From an eccentric alien to an animated blue genie, a loveable British nanny and a fatherly therapist, actor Robin Williams used his multi-layered brand of comedy to make people laugh.

But behind the boyish exuberance and hyperactive energy was a sensitive man who lived to perform in front of a crowd, friends and colleagues of the actor said following his death from an apparent suicide on Monday.

While the Oscar-winner's death came as a bolt from the blue to the public, the actor's longtime friend and fellow comedian Bob Zmuda said he was not "totally shocked" by the news.

Zmuda, the founder of the charity Comic Relief for which the actor had helped raise more than $70 million, said that Williams had difficulty connecting with people off-stage and that even those who knew him well were unaware of how severe his depression was.

WATCH: Fans gather at film sets of Robin Williams's popular movies to pay tribute​

"Sometimes I would meet him one-on-one, he would be so uncomfortable, he really had no social skills, and that's probably one of the reasons he needed to be on stage," Zmuda said. "That was his life blood, that was his psychological imperative, and without that, he was pretty lost."

Budd Friedman, the founder of The Improv comedy clubs, last saw Williams about a year ago when the actor dropped by a club to perform a short routine.

Friedman said in the 35 years he had known Williams, he had never seen any glimpse of his suffering.

"I don't know too many people who were that aware of that darkness in him," Friedman said.

Hours after Williams died on Monday, his representative Mara Buxbaum revealed, with unusual candor by Hollywood standards, that the actor and comedian had suffered from severe depression in recent months.

Comedy and depression have often been interwined, from Jim Carrey to Louis CK, Sarah Silverman and Williams's own idol, Jonathan Winters, who died last year.

"There are a lot of comedians who come from the background of tragedy. Comedians have a lot of demons," said Jamie Masada, founder of The Laugh Factory comedy club where Williams often performed his stand-up routines.

After seeing the inner struggles of comedians such as Richard Jeni and Richard Pryor, Masada hired psychiatrists to provide treatment for the acts performing at his clubs.

"(Therapy) helps in many ways," said Masada. "It's not the cure, but at least I'm not sitting back watching them hurt."

Zmuda said Williams appeared upbeat and excited in a new marriage at their last meeting during Williams's Broadway debut in 2011's "Bengal Tiger at the Baghdad Zoo."

Zmuda said the "Mrs. Doubtfire" star had faced challenges in recent years as well, especially after undergoing open-heart surgery in 2009 and when his TV comedy "The Crazy Ones" was cancelled in May after one season.

"I think the heart attack and his health, his age, the fact that he did not have that same ability, the show being cancelled, all of that affected him," Zmuda said.

"The pressure on you when you are rich and famous and you still have those problems of depression, it's almost like you have no escape," he added.

Williams had last visited The Laugh Factory in Hollywood about nine months ago, Masada said, delivering a half-hour stand-up set.

Williams received not just a standing ovation but also screaming fans, showing his enduring appeal within the comedy set, Masada added.